Cartilage is a tissue composed by a cellular component, chondrocytes, and by an extra-cellular matrix typically rich in collagen type II and highly sulphated high molecular weight proteoglycan aggregates. The latter property confers cartilage its peculiar histochemical characteristics that are: strong staining with Alcian blue at low pH (from 0.2 to 2.5) and metachromacy with Toluidine blue and Safranin O. The abundance of type II collagen, link protein, and proteoglycan aggrecan, along with the presence of minor collagens such as type IX and type XI collagen are hallmarks of cartilage tissue.
In post-natal mammals, cartilage contributes to the structure of several organs and systems like the articular surface of diarthrodial joints and other joint-associated structures (such as menisci), the ear, the nose, the larynx, the trachea, the bronchi, structures of the heart valves, part of the costae, synchondroses, entheses etc. In some of the mentioned locations (e.g. entheses, the annulus fibrosus of the intervertebral disks, in the menisci, insertion of ligaments etc.) for the abundance of collagens (mostly type I collagen) and the peculiar distribution of the fibrous bundles it is called fibrocartilage. In other locations (e.g. the pinna of the ear, epiglottis etc.) it is particularly rich in elastin and it is called elastic cartilage. In all the other structures, for its semi-transparent, clear aspect it is called hyaline cartilage.
During embryogenesis cartilage has a role in the development of long bones. Mesenchymal cells aggregate and differentiate to form cartilage anlagen, which provide the mold of the future long bones. These cartilage templates in development evolve, undergo endochondral bone formation through a cascade of events including chondrocyte hypertrophy, vascular invasion, mineralization, and are eventually replaced by bone except for a thin layer at the extremities of the bone elements that will differentiate into the articular surface of diarthrodial joints. In these locations cartilage tissue remains hyaline for all the life-span of the individual. With ageing, articular cartilage is well known to undergo a process of senescence, affecting its mechanical properties and its intrinsic resilience.
Joint surface defects can be the result of various aetiologies such as inflammatory processes, neoplasias, post-traumatic and degenerative events etc. Whatever the cause, the mechanisms of repair and of subsequent evolution are largely common.
Osteochondral (or full-thickness) articular surface defects include damage to the articular cartilage, the underlying subchondral bone tissue, and the calcified layer of cartilage located between the articular cartilage and the subchondral bone. They typically arise during severe trauma of the joint or during the late stages of degenerative joint diseases, e.g. during osteoarthritis. These lesions disrupt the congruence between the joint surfaces and therefore can lead to OA, which can be painful and severely limit the joint function. Osteochondral defects can rely on an extrinsic mechanism for repair. Extrinsic healing uses mesenchymal elements from subchondral bone to participate in the formation of new connective tissue. The repair tissue, however, often consists of fibrocartilage or fibrous tissue. This scar tissue does not share the same biomechanical properties as hyaline cartilage and eventually degenerates with the development of osteoarthritis.
Superficial or partial-thickness injuries of the articular cartilage that do not penetrate the subchondral bone can only rely on an intrinsic mechanism for repair. Chondrocytes adjacent to the injured surfaces proliferate and increase the deposition of extracellular matrix. Despite these attempts at repair, there is no appreciable increase in the bulk of cartilage matrix and the repair process is rarely effective in healing the defects. Although initially sometimes painless, partial-thickness defects often degenerate into osteoarthritis of the involved joint.
Repair of articular cartilage defects with suspensions of chondrocytes has been carried out in a variety of animal models (Brittberg et al. (1996) Clin. Orthop. (326):270-83) and is now employed in humans (Brittberg et al. N Engl J Med. 1994 Oct. 6; 331(14):889-95). Autologous chondrocytes obtained from an unaffected area of the joint are released, expanded in vitro in the presence of autologous serum and subsequently injected under a periosteal flap sutured to cover the cartilage defect. This procedure has led to a proven at least symptomatic amelioration. This conceptually promising approach has still wide margins for improvement, since it is known that in vitro expansion of chondrocytes results, after a limited number of cell divisions, in a loss of their phenotypic stability (as defined by the ability of chondrocytes to form hyaline cartilage in vivo) making the cell suspension to be injected unreliable.
Three alternative approaches have been developed in an attempt to improve the success rate in treating mammalian articular cartilage defects. In the first approach, synthetic carrier matrices are impregnated with allogeneic chondrocytes and then implanted into the cartilage defect where they hopefully produce and secrete components of the extracellular matrix to form articular cartilage at the site of the defect. A variety of synthetic carrier matrices have been used to date and include three-dimensional collagen gels (e.g. U.S. Pat. No. 4,846,835), reconstituted fibrin-thrombin gels (e.g. U.S. Pat. Nos. 4,642,120; 5,053,050 and 4,904,259), synthetic polymer matrices containing polyanhydride, polyorthoester, polyglycolic acid and copolymers thereof (U.S. Pat. No. 5,041,138), and hyaluronic acid-based polymers. Once a mitotically expanded population of chondrocytes is obtained, the cells can be implanted either back into the same subject from which their parent cells were originally derived (autologous implantation), or into a different subject (heterologous implantation). In addition, heterologous implantation may use chondrocytes obtained from a related or unrelated individual of the same species (allogeneic), or from a different species (xenogeneic). Alternatively, chondrocytes may be obtained from an established, long-term cell line that is either allogeneic or xenogeneic.
The introduction of non-autologous materials into a patient, however, may stimulate an undesirable immune response directed against the implanted material, leading to potential rejection of the newly-formed and engrafted cartilage tissue. In addition, heterologous implantation risks the transmission to the subject of infectious agent(s) present in the tissue or cell line. Neo-cartilage may be formed around the periphery of the implant thereby preventing integration of the implant into the cartilage defect. Monitoring the formation and development of the resulting synthetic cartilage in situ is difficult to perform and usually involves an arthroscopic or open joint examination. Furthermore, implants containing synthetic polymer components may be unsuitable for repairing large cartilage defects since polymer hydrolysis in situ inhibits the formation of cartilage and/or its integration into the defect.
In the second approach, the defect is filled with a biocompatible, biodegradable matrix containing chemotactic and mitogenic growth factors to stimulate the influx of chondrocyte progenitor cells into the matrix in situ. The matrices optimally contain pores of sufficient dimensions to permit the influx into, and proliferation of the chondrocyte progenitor within the matrix. The matrix also may contain differentiating growth factors to stimulate the differentiation of chondrocyte progenitor cells into chondrocytes which in turn hopefully secrete extracellular matrix components to form cartilage at the site of the defect in situ (e.g. U.S. Pat. Nos. 5,206,023 and 5,270,300 and EP-A-530,804). This approach however results in problems similar to those associated with the first approach hereinabove. Furthermore there is no data so far that articular cartilage contains chondrocytic progenitors available for partial thickness defect repair.
In the third approach, chondrocytes may be cultured and expanded in vitro thereby to form synthetic cartilage-like material that is implanted subsequently into the cartilage defect. This has the advantage over the previous methods in that the development of the synthetic cartilage material may be monitored, through morphological, biochemical, and molecular characterisation, prior to implantation. Chondrogenic cells may be expanded in either an anchorage-dependent or an anchorage-independent culture system. In the latter, chondrogenic cells may be cultured as colonies within an agarose gel. Heretofore, only small pieces of cartilage tissue of undefined shape have been prepared using this manner. Furthermore, the resulting cartilage remains embedded within a gel matrix making it less suitable for implantation into mammals. Alternatively, in another anchorage-independent method, chondrocytes may be cultured as colonies in suspension culture. However the resulting particles containing synthetic cartilage-like material are usually small and of undefined shape, and do not integrate with each other and with the surrounding cartilage within the defect. This makes them unsuitable for implantation and repair of a predetermined articular cartilage defect.
In the anchorage-dependent method, primary cultures of chondrogenic cells isolated from primary tissue are grown as monolayer attached to the surface of a cell culture flask (e.g. U.S. Pat. No. 4,356,261). The primary cells derived directly from explant tissue remain capable of producing and secreting extracellular components characteristic of natural cartilage, specifically type II collagen and sulphated proteoglycans. However, it is well known that during in vitro expansion as monolayers, chondrocytes dedifferentiate and lose their ability to form hyaline cartilage in vivo. Until now it has not been possible to prepare large patches of articular cartilage from small pieces of biopsy tissue using the anchorage-dependent procedures of U.S. Pat. No. 4,356,261.
In order to solve the above problems, U.S. Pat. No. 5,723,331 provides a method for preparing in vitro large quantities of synthetic cartilage from small samples of biopsy tissue which, based on the discovery that chondrogenic cells may be isolated from a variety of tissues, e.g. pre-existing cartilage, perichondrial tissue or bone marrow, and expanded in vitro prior to cartilage formation, includes first seeding denuded (i.e. isolated from an enzymatically or mechanically disaggregated tissue) chondrogenic cells, proliferated ex vivo, into a pre-shaped well having a cell contacting, cell adhesive surface, and then culturing the proliferated chondrogenic cells in the well for a time sufficient to permit the cells to secrete an extracellular matrix thereby to form a three-dimensional, multi cell-layered patch of synthetic cartilage. This approach does not yield an optimal integration between the implant and the surrounding cartilage. Thus far there is no evidence on the phenotypic stability of cells in such preparations.
The use of mesenchymal cells has also been proposed for cartilage repair. Mesenchymal cells are a potential alternative source of cartilage-producing cells. They are generally recognised as pluripotent cells capable of dividing many times to produce progeny cells that can eventually give rise to many tissues, including skeletal tissues such as cartilage, bone, tendon, ligament, marrow stroma and connective tissue. By definition, they can undergo many more divisions. Chondro/osteoprogenitor cells, which are bipotent with the ability to differentiate into cartilage or bone, were isolated from bone marrow (e.g. in U.S. Pat. No. 5,226,914), and subsequently from muscle, heart and granulation tissue. Pluripotency is demonstrated using different culture conditions and adding more or less specific inducers, which elicit differentiation of the stem cells into chondrocytes (cartilage), osteoblasts (bone), myotubes (muscle), adipocytes (fat).
It would be highly desirable to have progenitor cells which are easily obtained such as by muscle biopsy, cultured to yield large numbers, and can be used as a source of chondrocytes or osteoblasts or myocytes. However, the same pluripotency that makes them attractive, conveys the risk of metaplastic differentiation. In other words there is the risk that they could differentiate in an undesired direction (e.g. bone or fat within a cartilage defect). In U.S. Pat. Nos. 5,226,914 and 5,197,985 the cells were absorbed into porous ceramic blocks and implanted, yielded primarily bone. However, U.S. Pat. No. 5,906,934 discloses that under very specific conditions mesenchymal stem cells in a suitable polymeric carrier (such as polyglycolic acid mesh) implanted into a cartilage and/or bone defect will differentiate to form cartilage and/or bone, as appropriate. Also U.S. Pat. No. 5,919,702 discloses chondrocyte progenitor cells isolated from umbilical cord sources, e.g. from Wharton's jelly, and cultured so as to give rise to chondrocytes that can produce cartilage tissue. Also in another attempt to avoid the drawbacks of current cartilage and bone repair techniques which cause bleeding and involve the use of mechanically weak non self-derived material, U.S. Pat. No. 5,866,415 suggests treating cartilage or bone defects with a biological material obtained by attaching in vitro cartilage or bone forming cells to a periosteum of sufficient size to accommodate the defect.
WO/96/41523 and WO96/41620 describe the use of FGFR3 as a marker for mesenchymal skeletal progenitor cells. Such cells do not show a stable phenotype. To initiate differentiation of these cells factors may be added to the cells or in situ, for example an FGF9 antagonist. As indicated above the use of progenitor cells for implantation in the body is counter-indicated due to the danger of metaplastic differentiation.
Transforming growth factor-beta (“TGF-β”) refers to a family of related dimeric proteins which regulate the growth and differentiation of many cell types. Members of this family include TGF-β1, TGF-β2, TGF-β3, TGF-β4, TGF-β5, morphogenic proteins (“MP”) such as MP-121 and MP-52, inhibins/activins (such as disclosed in EP-A-222,491), osteogenic proteins (“OP”), bone morphogenetic proteins (hereinafter denoted “BMP”), growth/differentiation factors (“GDF”) such as GDF-1, GDF-3, GDF-9 and Nodal. TGF-β was first characterised for its effects on cell proliferation. It both stimulated the anchorage-independent growth of rat kidney fibroblasts and inhibited the growth of monkey kidney cells. TGF-β family members have been shown to have many diverse biological effects, e.g. they regulate bone formation, induce rat muscle cells to produce cartilage-specific macromolecules, inhibit the growth of early haematopoietic progenitor cells, T cells, B cells, mouse keratinocytes, and several human cancer cell lines. TGF-β family members increase the synthesis and secretion of collagen and fibronectin, accelerate healing of incisional wounds, suppress casein synthesis in mouse mammary explants, inhibit DNA synthesis in rat liver epithelial cells, stimulate the production of bFGF binding proteoglycans, modulate phosphorylation of the epidermal growth factor (“EGF”) receptor and proliferation of epidermoid carcinoma cells and can lead to apoptosis in uterine epithelial cells, cultured hepatocytes and regressing liver. TGF-β's can mediate cardio-protection against reperfusion injury by inhibiting neutrophil adherence to endothelium and protect against experimental autoimmune diseases in mice. On the whole, proteins of the TGF-β family are multifunctional, active growth factors and also have related biological activities such as chemotactic attraction of cells, promotion of cell differentiation and tissue-inducing capabilities. Differences in their structure and in their affinity for receptors lead to considerable variations in their exact biological function.
In contrast to the foregoing reports of the ability of TGF-β to induce the production of cartilage-specific macromolecules in muscle cells and chondrocytes, TGF-β was found to act synergistically with fibroblast growth factor to inhibit the synthesis of collagen type II by chicken sternal chondrocytes and in rat chondrocytes. In fact, TGF-β, has emerged as the prototypical inhibitor of the proliferation of most normal cell types in vitro as well as in vivo, exhibiting a remarkable diversity of biological activity. TGF-β1 has been purified from human and porcine blood platelets and recombinant TGF-β1 is currently available.
Among the sub-family of BMPs, the structures of BMP-1 through BMP-15 have previously been elucidated. The unique inductive activities of these proteins, along with their presence in bone, suggests that they are important regulators of bone repair processes and may be involved in the normal maintenance of bone tissue. Recently, the BMP-12-related subfamily of proteins, including BMP-13 and MP52 (see e.g. WO93/16099 and U.S. Pat. No. 5,658,882), was shown to be useful in compositions for the induction of tendon/ligament-like tissue formation and repair. U.S. Pat. No. 5,902,785 discloses that BMP-12 related proteins are particularly effective for the induction of cartilaginous tissue and that BMP-9 is useful for increasing proteoglycan matrix synthesis and therefore for the maintenance of cartilaginous tissue. It also describes compositions comprising a BMP-12 related protein and additionally including one or more TGF-β superfamily member proven to be osteogenic, preferably BMP-2, -4, -5, -6 and/or BMP-7 as useful for the regeneration of multiple tissue types (for example at the interface or junction between tissues) and especially useful for the treatment of articular cartilage, in which the articular surface, cartilage, subchondral bone and/or tidemark interface between cartilage and bone need to be repaired. The same patent further describes compositions comprising a BMP-12 related protein together with a protein useful for the maintenance of chondrocytes or cartilaginous tissue such as BMP-9, the said compositions being especially useful for the induction and maintenance of cartilaginous tissue at a site in need of cartilage repair such as an articular cartilage defect.
WO96/14335 discloses, using mRNA prepared from newborn articular cartilage, the isolation of two members of the BMP family, designated Cartilage-derived morphogenetic proteins-1 and -2 (CDMP-1, CDMP-2). Storm et al. (1994) in Nature 368, 639-43 and Chang et al. (1994) in J.Biol.Chem. 269, 28227-34 independently established that CDMP-1 mapped close to the brachypodism locus on chromosome 2 in mice and might be involved in the brachypodism phenotype. Also the expression patterns of CDMP's suggests an important role for these genes in joint morphogenesis. WO98/59035 also discloses a method of maintaining a cartilaginous phenotype in chondrocytes in vitro, comprising culturing the chondrocytes in serum-free medium containing a CDMP and/or BMP.
Table 1 summarises the BMP subfamily members according to Reddi A H, Nature Biotechnol. 1998, 16:247-52.
TABLE 1The BMP family in mammalsBMPBMPsubfamilyGeneric namedesignationBMP 2/4BMP-2ABMP-2BMP-2BBMP-4BMP 3OsteogeninBMP-3Growth/differentiation factor 10BMP-3BOp-1/BMP-7BMP-5BMP-5Vegetal related-1 (Vgr-1)BMP-6Osteogenic Protein-1 (Op-1)BMP-7Osteogenic Protein-2 (Op-2)BMP-8Osteogenic Protein-3 (Op-3)BMP-8BGrowth/differentiation factor 2 (GDF-2)BMP-9BMP-10BMP-10Growth/differentiation factor 11 (GDF-11)BMP-11GDF-5, 6, 7Growth/differentiation factor 7 (GDF-7) orBMP-12cartilage-derived morphogenic protein-3(CDMP-3)Growth/differentiation factor 6 (GDF-6) orBMP-13cartilage-derived morphogenic protein-2(CDMP-2)Growth/differentiation factor 5 (GDF-5) orBMP-14cartilage-derived morphogenic protein-1(CDMP-1)BMP-15BMP-15
Other families of growth factors have been shown to play a role in cartilage formation/differentiation. Among them the fibroblast growth factors (FGFs) are a family of polypeptide growth factors involved in a variety of activities. One of their receptors, FGF receptor 3 (FGFR-3) (Keegan K. et al., 1991 Proc. Nat. Acad. Sci. 88: 1095-99), is known to play a crucial role in chondrogenesis. Point mutations in the fgfr3 gene resulting in a ligand-independent constitutively active protein (which means that the FGF signalling is always active also in the absence of the ligand) cause skeletal abnormalities as achondroplasia and thanatophoric dysplasia.
As already outlined above, although autologous chondrocyte transplantation (“ACT”) is becoming a widely accepted technique for repair of joint surface defects (“JSD”) it still presents some drawbacks. More in detail, this procedure implies in vitro expansion—in the presence of autologous serum—of autologous chondrocytes obtained from an uninvolved area of the joint surface, followed by the implantation of the chondrocyte suspension under a periosteal flap sutured to seal the joint surface defect. Cell expansion is necessary to obtain from a small cartilage biopsy a number of cells sufficient to repair the cartilage defect. Expansion in monolayer result in the loss of phenotypic traits in chondrocytes (Benya and Shaffer. 1982, Cell 30:215-24). To date, however, it is not known how far it is possible to expand chondrocytes without hampering their phenotypic stability and therefore their capacity to form stable hyaline cartilage in vivo, resistant to vascular invasion and endochondral bone formation. Other factors that can affect the capacity of chondrocytes to form cartilage in vivo are the culture conditions, and several factors dependent on the donor such as age and pre-existing joint or systemic diseases. At the end of cell expansion the chondrocyte population is composed of some cells that retain their phenotypic stability, and others that still can proliferate but will not anymore contribute to cartilage repair. To obtain a consistent cell suspension for ACT, it is desirable to determine which is the actual capacity of the cells to form cartilage in vivo and, if necessary, to select stable chondrocytes within the expanded cell population. The importance of this issue is underscored by the large variability in the quality of the repair tissue obtained in a large series (Peterson et al. Clin Orthop [374], 212-234. 2000.) consisting of a range going from hyaline-like cartilage to fibrocartilage to no signs of repair.
Chondrocytes are the only normal skeletal cells known to grow anchorage-independent in agarose cultures (Benya and Shaffer. 1982, Cell 30:215-224). This culture system allows a recovery of some of the phenotypic traits that are lost with expansion in monolayer (Benya and Shaffer. 1982, Cell 30:215-224). The expression of type 2 collagen and the capacity to grow and rescue phenotypic traits in agarose culture, are good assays to evaluate chondrocyte differentiation and the potential to differentiate respectively. However they do not measure the capacity of chondrocytes to form cartilage in vivo.